Field of the Invention
The present invention relates to a sensor in which terminal wires extending from, for example, a temperature-sensitive portion of a temperature sensitive element are joined by welding to conductive wires (e.g., lead wires) each formed of a plurality of conductors and to a method of producing the sensor. Particularly, the present invention relates to a sensor, such as a vehicle-mounted temperature sensor or a high temperature sensor used for exhaust gas measurement in a general-purpose stationary engine etc., which can be preferably used even under use conditions under which vibrations are applied to the main body of the sensor, the peripheries of its lead wires, etc. The present invention also relates to a method of producing this sensor.
Description of Related Art
Temperature sensors have been used to measure the temperature of, for example, exhaust gas from engines, and temperature sensors having various structures have been proposed.
For example, Patent Document 1 discloses a technique for solder-joining a terminal wire extending from a thermistor element covered with an insulator to a plurality of conductors protruding from an end of an insulator-coated wire. Patent Document 2 discloses a technique for joining each of terminal wires extending from a thermistor element through insulator tubes and to a plurality of conductors forming a lead wire by welding within a temperature sensor on its rear end side.
A problem arises when a terminal wire is joined to a plurality of conductors of a lead wire by resistance welding, in that it is not easy to control the welding current. Specifically, when the lead wire is formed of a plurality of small-diameter conductors, the conductors may melt and break during welding if the welding current is large. When the welding current is set to be low to prevent this problem, welding fails to be performed completely, so that the welded portions may be separated from each other.
In order to weld a plurality of loose conductors to one plate-shaped or rod-shaped terminal wire in a reliable manner, welding must be repeated about 4 to 5 times, and, disadvantageously, the welding operation requires much time and effort.
In view of the above, there has been developed a technique for integrally joining a terminal wire to a conductive wire composed of a plurality of conductors, which are twisted together, in an easy and reliable manner (see Patent Document 3). Specifically, the plurality of conductors are fusion-bonded and integrated in advance by first welding, and then the conductive wire composed of the plurality of (integrated) conductors is welded to the terminal wire (second welding).
With this technique, the terminal wire is connected by the second welding to the conductive wire composed of the plurality of small-diameter conductors that have been integrated by the first welding so as to be substantially thickened as compared to the individual conductors. Therefore, the welding current in the second welding can be appropriately set.